The present invention is directed to a fuel cell bipolar separator plate and current collector assembly, and, more particularly, to an improved fuel cell bipolar separator plate and collector assembly having improved manufacturability and cooling, and a corresponding method of manufacture.
A fuel cell stack consists of multiple planar cells stacked upon one another, to provide an electrical series relationship. Each cell is comprised of an anode electrode, a cathode electrode, and an electrolyte member. A device known in the art as a bipolar separator plate, an interconnect, a separator, or a flow field plate, separate the adjacent cells of a stack of cells in a fuel cell stack. The bipolar separator plate may serve several additional purposes, such as mechanical support to withstand the compressive forces applied to hold the fuel cell stack together, providing fluid communication of reactants and coolants to respective flow chambers, and to provide a path for current flow generated by the fuel cell. The plate also may provide a means to remove excess heat generated by the exothermic fuel cell reactions occurring in the fuel cells.
Each individual cell produces about 0.5-1.0 volts DC. Electrical current output capacity is based upon the area of the fuel cell electrodes times the current density capacity of the cell. Maximum achievable current density, measured in amps/cm2, varies from fuel cell type to fuel cell type. Therefore, the quantity of cells (voltage), the area of the cells (current), and the current density of the cells determine the kW output capacity of a fuel cell stack. In order to achieve output capacities suitable for distributed generation, the fuel cell stack must output a minimum of about 2-10 kW for residential applications, to about 50-100 kW for light commercial/industrial applications. In these scenarios, the fuel cell stack may consist of from about fifty to in excess of one-hundred-and-fifty cells having an area of about 200 cm2 to about 3000 cm2.
Another important measure of a fuel cell stack, in addition to current density, is its volumetric power density. High volumetric power density is desirable for both stationary and transportation applications of fuel cells. Volumetric power density is measured as the watt density per cm2 of an individual cell times the quantity of cells per linear centimeter of stack height. Therefore, it is desirable to design thin cells to achieve high volumetric power density.
While current density is more a function of the individual fuel cell type, volumetric power density is mostly a function of the physical design of the fuel cell components and the design of the bipolar separator plate.
The design of bipolar separator plates in the prior art has been driven by many wide ranging factors, such as cell chemistry, reactant flow configurations, material selection, system pressurization, operating temperature, system cooling requirements, and intellectual property considerations.
However, there are several common characteristics of bipolar separator plate design. Prior art bipolar separator plates have typically been produced in a discontinuous mode utilizing highly complex tooling that produces a plate with a finite cell area. Alternatively, prior art plates having a finite area may be produced from a collection of a mixture of discontinuously and continuously manufactured sheet-like components that are assembled to produce a single plate possessing a finite cell area. U.S. Pat. No. 6,040,076 to Reeder teaches an example of a Molten Carbonate Fuel Cell (MCFC) bipolar separator plate produced in this fashion, where plates are die formed with a specific finite area of up to eight square feet. U.S. Pat. No. 5,527,363 to Wilkinson et. al. teaches an example of a Proton Exchange Membrane Fuel Cell (PEMFC) xe2x80x9cembossed fluid flow field plate,xe2x80x9d also die formed with a discrete finite area. U.S. Pat. No. 5,460,897 to Gibson et. al. teaches an example of a Solid Oxide Fuel Cell (SOFC) interconnect, also produced having a finite area. Bipolar separator plates produced with a discontinuous finite area do not enjoy the advantages of continuous production methods such as are commonly used to produce the electrodes and electrolyte members of the fuel cell. Continuous production methods provide cost and speed advantages and minimize part handling. Continuous production using what is known as progressive tooling allows the use of small tools that are able to produce large plates from sheet material. The plate described in Reeder is able to be produced in a semi-continuous fashion, but requires tooling possessing an area equivalent to that of the finished bipolar plate area. The plate described in Reeder requires separately produced current collectors for both the anode and cathode. These current collectors may be produced in a continuous fashion. However, the resultant assembly is material intensive, comprised of three sheets of material. The area of the plate created by the design is fixed and unalterable unless retooled.
Production methods that utilize molds to produce plates from non-sheet material, such as injection molding with polymers, are wholly unable to stream the production process in a continuous mode. As a result, discontinuous production methods require complex tooling and are speed limited. Complex tooling further inhibits design evolution due to the costs associated with replacing or modifying the tools.
Another commonality among the bipolar separator plate designs of the various fuel cell types is the material of construction. Although carbon graphite, polymers, and ceramics are common examples of the materials of choice for the bipolar separator plate of the various fuel cell types, sheet metal can also be found as an example of the material of choice for each of the fuel cell types in the prior art literature. For example, Reeder teaches a metallic MCFC bipolar separator plate. U.S. Pat. No. 5,776,624 to Neutzler teaches a metallic PEMFC bipolar separator plate. Gibson teaches a metallic SOFC bipolar separator plate. U.S. Pat. No. 6,080,502 to Nolscher et. al. teaches a metallic bipolar separator for fuel cells and denotes fuel cells as including Phosphoric Acid Fuel Cell (PAFC) and Alkaline Fuel Cell (AFC). Sheet metal, or metal foil, permits the application of high-speed manufacturing methods such as continuous progressive tooling. Metallic bipolar separator plates for fuel cells further provide for high strength and compact design.
A third commonality of bipolar separator design can be found in the various methods to provide a means to cool the fuel cell. Although some fuel cell stack designs elect to disperse this critical function via dedicated cooling plates at intervals of several cells, or within a wholly separate cooling section, examples of a bipolar separator plate from each fuel cell type can be found to include an integral coolant chamber. These chambers may be designed for gaseous coolant, liquid coolant, or endothermic fuel reforming. Providing a coolant chamber to each individual bipolar separator plate presents engineering and design challenges. Specifically, plate thickness and reactant/coolant manifolding are impacted by the addition of a coolant chamber. The impact on plate thickness can be minimized by using a liquid coolant that possesses a greater heat carrying capacity than do gaseous coolants such as air. Neutzler teaches a xe2x80x9ccoolant flow passagexe2x80x9d centrally located between two outer metallic sheets. Nolscher teaches a xe2x80x9ccooling medium distribution ductxe2x80x9d also located between two metallic sheets. In both cases, the design utilizes two opposing sheets of material die-formed with a plurality of grooves, or ribs. The cooling chamber is formed when the maximum elevation of one sheet rests on the maximum depression of the subsequent sheet. Both sheets are structural members of the bipolar plate and therefore must be of sufficient strength and robustness to withstand the compressive sealing force applied to the assembled fuel cell stack. U.S. Pat. No. 5,795,665 to Allen teaches a xe2x80x9creforming compartmentxe2x80x9d within an MCFC bipolar separator plate formed when the maximum elevation of a dimpled single-piece bipolar separator rests on the maximum depression of a dimpled subassembly of active components and current collector with a flat sheet barrier disposed between the two components. The resulting chamber is equipped with a reforming catalyst for endothermic stream reforming of fuel.
A fourth commonality of bipolar separator design can be found in the various reactant flow and reactant manifold configurations. The existing alternatives for flow configuration are co-flow, counter-flow, and cross-flow, as well as variations utilizing serpentine flows. The existing designs for reactant and coolant manifolds are internal, external, or a combination of internal and external. Manifolding the fuel, oxidant and coolant to provide uniform flow to the surfaces of the bipolar separator plate contributes to the overall design complexity. Neutzler teaches parallel flow of reactants and cross-flow of the coolant via manifold openings surrounding the periphery of the plate assembled from three components. Nolscher teaches parallel flow of reactants and coolant manifolded via xe2x80x9cdistribution ductsxe2x80x9d that extend almost entirely along opposing edge areas of the bipolar separator plate assembled from two components. The distribution ducts of Nolscher are said to provide more uniform flow of flow streams than that which is provided from xe2x80x9cpoint-likexe2x80x9d inlets. However, this design would be limited in practical applications employing cell areas with a large dimension perpendicular to the direction of the flow streams in the active area. This design is not discussed in Nolscher as being produced in continuous form or consisting of repeated sections.
A fifth commonality of bipolar separator design can be found in the various methods used to achieve efficient packaging of the plate to yield a thin structure promoting high volumetric power density. Often, however, volumetric power density is difficult to achieve when attempting to utilize the optimum configurations for the latter commonalties of material of construction, cell cooling, and reactant/coolant flow and manifold configuration.
A need exists for a bipolar separator plate produced in continuous mode from sheet material with a reactant/coolant flow and manifold configuration having a thin structure with low material content, a coolant flow path, uniformity of reactant/coolant flow streams, that can be produced in high volume at high speed, and that is applicable to several types of fuel cells.
The need is further exemplified when contemplating a staged-array of fuel cells bottomed by fuel cells. Clearly, in this scenario, an MCFC stack staged with an SOFC stack would greatly benefit from uniformity in design of the bipolar separator with respect to manifolding interconnections and general system packaging and design.
Manufacture of the bipolar separator plate of the present invention is accomplished by producing repeated finite sub-sections of a bipolar separator plate in continuous mode. The plate may be cut to any desirable length in multiples of the repeated finite sub-section and processed through final assembly, or recoiled for further processing.
The structure of the separator plate that creates flow channels and manifolds is stretch-formed into finite sub-sections by what is known in the art as progressive tooling. Progressive tooling is an efficient means to produce complex stampings from a series of low-complexity tools, or, as a means to produce a product whose area is substantially larger than the tool that is utilized. As a result, the bipolar separator plate of the present invention produced in this manner possesses modularity not found in conventional discontinuous bipolar separator plate designs. The scaleable cell area of such a separator plate provides responsiveness to a wider range of fuel cell applications, from residential to light commercial/industrial to automotive, without deviating from the underlying geometries. Though fuel cell stacks clearly are scaleable by altering the quantity of cells comprising the stack of cells, it is advantageous to efficiently alter the area of the cells as well. As is well known in the art, cell count determines stack voltage while cell area determines stack current.
What is particularly advantageous with the present invention is that the repeated finite sub-sections of the continuously produced bipolar separator plate do not require discontinuity of the electrodes and electrolyte member of the fuel cell. Many of the conventional designs of the prior art bipolar separator designs are quite capable of continuous, progressively tooled, manufacture. However, all prior art designs would require discontinuity of the electrodes and electrolyte members in order to properly fit the resultant repeated finite sub-sections. Many prior art designs are incapable of continuous progressive tooling due to the nature of the fuel, oxidant, and coolant manifolding and flow pattern designs.
The present design provides for parallel flows of reactants and coolant within each sub-section. The parallel flows are arranged perpendicular to the direction of manufacture. The flow channels are comprised of parallel ribs stretch-formed into the main body of the bipolar separator plate. Stretch-forming is known in the art as a means to form sheet materials into complex shapes and is distinguished from draw forming by the avoidance of material from outside the confines of the tool flowing into the tool during the stamping process. The reactant flow format of the design may be any combination of co-flow or counter-flow to satisfy the desired operational characteristics of the fuel cell type. Each sub-section is provided with at least one pair of internal manifolds and at least one pair of external manifolds within the edge sealing areas of the plate. The method of manufacture results in the manifolds being repeated along the opposing edge sealing areas. This feature promotes uniformity of reactant and coolant flow distribution. The manifolds of each sub-section and the area of each sub-section are stoichiometricly matched to provide the desired electrochemical reaction rates and pressure-drops within that particular sub-section.
These and additional features and advantages of the invention disclosed here will be further understood from the following detailed disclosure of preferred embodiments.